I Love You To Bits: Female Sex Cannibalism and Male Counter-Adaptation in the Redback Spider

The redback spider (Latrodectus hasselti) is a nocturnal venomous spider found in Australia which displays sexual cannibalism during mating.

Figure 1: A female redback spider (Latrodectus hasselti)

Female Sex Cannibalism and Post-Copulatory Paternity Choices

For the female, killing the male is not such a big deal because she can easily raise the offspring on her own and is able to store the sperm in her reproductive tract called spermathecae for as long as two years to fertilize its eggs later.

Females mate with multiple males and research has shown they have evolved two separate sperm storage organs, allowing them to make post-copulatory paternity choices. This post-copulatory female choice has been termed ‘cryptic female choice’, because any differential selection of sperm occurs at the hidden level of the gamete.

Some argue that any case of polyandry, where females have multiple mates, with sperm survival between insemination and fertilisation, allows ‘cryptic female choice’, because paternity will be influenced by the male with the ‘best’ sperm. Under these conditions, females can (indirectly) choose which male is the successful fertiliser on the basis of spermatozoal, and hence cryptic, traits.

In addition females might have evolved a non-random sperm-storage system that encourages the last male-to-mate to be the successful fertilizer. This pattern of ‘last male sperm precedence’ is common in insects and allows a female to start reproducing quickly with the first male she meets, but if she meets a ‘preferred’ or ‘superior’ male further into her reproductive lifespan she can annul the representation of previous males’ sperm in storage and bias precedence towards the most recent Mr Right.

Figure 2: The separate, paired female sperm storage organs of the redback spider which allow cryptic female choice. Sperm are stored in the spermathecae (sp) following insemination through the copulatory duct (cd).

The Male Counter-Adaptation: An Abdominal Constriction in Courtship Dance

To attract a female, male redback spiders are known to engage in a complicated courtship dance, and if their performance is flawed, they are not just rejected they are eaten too. Nonetheless scientists believe that the male mating efforts are not in vain; that the male sacrifices so that the copulation can last longer allowing fertilization of a bigger number of eggs. It is also believed that once the female has eaten the male, it is more likely to reject other males.

Male spiders maximize paternity if they copulate twice with their cannibalistic mate. Therefore facilitating cannibalistic attack during their first copulation yields paternity benefits. In contrast, males that die after one copulation leave one storage organ empty and can lose 50% or more of their paternity to later-mating rivals. Sacrificial behaviour thus entails an evolutionary conflict for—sexual selection favours males that facilitate cannibalism at the first copulation, but ensures survival to the second copulation.

Therefore counter-adaptation by males, to maximise paternity after cannibalism, has led to the evolution of a novel male trait--an abdominal constriction that allows males to survive and mate with females for a second time, despite the substantial cannibalistic damage inflicted during the first copulation.

As they mate, the male is known to do a somersault, placing its abdomen just over the female’s mouth which then embarks on eating the latter. Females typically masticate the posterior dorsum of the male's abdomen during the first copulation.

A mid-dorsal abdominal constriction, which has been observed recently in redback males increases mobility and mating success following partial cannibalism. Constricted males that were wounded to simulate early cannibalism had higher endurance, greater survivorship, longer subsequent courtship and higher mating success than wounded males that were not constricted.

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